Power electronic circuits are used to control and condition electric power. For instance, power electronic circuits may be used to convert a direct current into an alternating current, to change voltage or current magnitude, to change the frequency of an alternating current, or to provide Volt-Ampere Reactive (VAR) power conditioning.
VAR power conditioning, also called reactive power conditioning herein, is of increasing importance in view of the growing levels of reactive loads on AC power distribution systems. Reactive power conditioning systems must function properly in the presence of voltage and current distortions on the power system.
Voltage and current distortions are largely attributable to a growing number of nonlinear loads in the electric utility power network. Typical nonlinear loads are computer controlled data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical, and communication equipment.
Nonlinear loads draw non-sinusoidal currents instead of purely sinusoidal currents drawn by conventional linear loads. As a result, currents at frequencies other than the fundamental power frequency flow through the predominantly inductive source impedance of the electric supply network.
Presently, there are static VAR compensators (SVCs) in operation which utilize computers to process line voltage data and which use solid state switches to switch compensating capacitors onto the power line to provide reactive power compensation. Unfortunately, reactive power conditioning by switching capacitors onto the power line may produce resonant conditions. Specifically, since the source impedance of a power system is inductive, a resonant frequency exists with the source inductance when a compensating capacitor is switched onto the power line. This can result in a large resonant current in the compensating capacitor, particularly if a significant voltage harmonic is present at the resonant frequency. If the resonant condition is sustained, it can lead to capacitor damage or even semiconductor device damage within the SVC.
Aside from the problem of resonant capacitor currents, SVCs have difficulties dealing with system harmonics. System harmonics can cause the thyristors of an SVC to prematurely turn-off as soon as the gate-drives to the thyristors are removed.
In each AC power cycle, the thyristor switches are triggered to turn on when their anode-to-cathode voltage is zero and is increasing in the forward direction. As long as the current through the thyristors is positive, it remains in the conducting state, even after the trigger signal is removed. This normally occurs for half of every AC cycle of the applied voltage. If the anode-to-cathode voltage applied across the thyristor is not a single-frequency sinusoid, or close approximation thereof, the thyristor will turn off prematurely when positive anode-to-cathode current is not maintained. As a result, the capacitor in series with the thyristor will not conduct for the required full half-cycle of the fundamental frequency. This results in providing less than the full amount of capacitive reactance.
Existing technology attempts to eliminate the premature shut-off of thyristors by triggering them either continuously throughout the cycle or by repetitive pulsing, thereby forcing the thyristors to stay on over the full 180 degree conduction period. This continuous gate drive methodology increases the gate drive power dissipation to undesirable or unacceptable levels. In addition, if a large magnitude of high frequency current is allowed to flow continuously, excessive rms heating of the switched capacitors results, leading to capacitor failure.
Thus, it would be highly desirable to provide an SVC which is immune from the problem of resonant capacitor currents, does not rely upon continuous thyristor gate drive triggering, and provides appropriate reactive power conditioning, even in the presence of premature thyristor turn-off.
The implementation of any power conditioning strategy relies upon firing the solid state switches of an SVC at a specific time in each cycle. If the thyristors are not switched at the correct moment, the compensating capacitor may produce transients on the power line. For correctly timed compensating capacitor switching to occur, the firing system must be synchronized with the power line fundamental frequency. In order for this synchronization to be accomplished, an approach well-known in the art is to determine, with hardware circuitry, the zero-crossings of the fundamental frequency of the line voltage or current. The problem with this widely-used technique is that an unambiguous determination of the zero-crossing point is difficult when system harmonics and resonances are present. In such a case, more than one zero-crossing may occur during each cycle of the fundamental frequency.
The presence of line harmonics is growing with the increasing use of solid state power conversion equipment. The harmonic problem is especially troublesome for single-phase AC circuits, because the information available for determining the zero crossings in a three-phase power system is not available in a single-phase system.
Hardware filters can be employed to reduce the measured harmonic content in a power line signal. However, hardware filters introduce waveform lag into the control system. This lag is proportional to the amount of harmonic content which must be filtered. Thus, the response time of the firing system may become limited in systems where significant harmonics are present. Attempting to reduce the lag in the hardware filter will cause the detection of multiple zero crossings and could cause a firing of the capacitor switch at the wrong point, with attendant undesirable transients or power circuit damage.
Thus, it would be highly desirable to develop a system for accurately determining the proper instant at which to activate the switches of an SVC, a capacitor bank, or other application of fast solid state switches. The switch firing system should not be sensitive to line harmonics and should not rely upon hardware filters. In addition, it would be highly desirable to develop a solid state switch firing system that does not rely upon external synchronization signals to identify the fundamental of the line signal. Finally, it would be highly desirable to develop a solid state switch firing system that can operate on a single-phase system.